1. Field of the Invention
The present invention relates to the field of active crystalline luminescent media. In particular, the present invention relates to doped active crystalline media for laser systems, tunable in the ultraviolet ("UV") to visible spectral range.
2. Background
Over the last fifteen years the laser community has shown an increasing interest and has shown some success in developing tunable laser systems. Interest in tunable laser systems has increased primarily because of the superior output characteristics of these laser systems, which give rise to numerous promising applications, such as use in ranging, lidar and optical communications, diagnostic and therapeutic medicine, photochemistry and photobiology, environmental monitoring, and kinetic and ultra-fast laser spectroscopy.
Active crystalline luminescent media usable for tunable laser systems may generally be classified into four different groups. The classification of a particular crystal within these four groups depends upon the particular crystal's working optical center. The working optical center is the structural feature of the crystal which provides the optical properties necessary for use in tunable laser systems.
The first group of active crystalline luminescent media is made up of impurity doped crystals. Crystalline media that fall into this group include crystals, such as chromium activated alexandrite, rare earth aluminum garnets, yttrium aluminum garnets, forsterite, complex fluorides (e.g. LiCaF, LiSaF, etc.) and titanium doped sapphire. Active crystalline media that fall within this first group obtain laser properties at the stage of crystal growth, when the melt is doped with a metal ion impurity.
The second group of active crystalline luminescent media is made up of color center crystals. Active crystalline media that fall within this second group include crystals having intrinsic, radiationally, additively or electrolytically induced F centers, which provide laser properties. Such crystals mainly include alkali-halide crystals, alkali-earth fluoride crystals and some oxide crystals (e.g., Al.sub.2 O.sub.3). Color center crystals are colorless and optically transparent in their primal state after crystal growth. They acquire laser properties only after additional crystal treatment. This additional treatment may include ionizing irradiation (e.g., irradiating with .gamma.-rays, x-rays, electrons, neutrons, etc.) or calcining in an alkali-metal vapor (i.e., additive coloration). The working optical centers of active crystalline media falling within this second group are complexes derived from F center defects in the crystal lattice. An F center defect is one of the simplest intrinsic point defects in the crystal structure.
The third group of active crystalline luminescent media includes crystals with complex optical dipoles caused by impurities adjacent to F center defects. The active crystalline media that fall within this third group may be divided into two subgroups. The first subgroup includes crystals having optical centers, which provide laser properties, that are represented by color centers, perturbed by neighboring impurities. Crystals that fall into this first subgroup include: F.sub.2.sup.+ * -F.sub.2.sup.+ centers, perturbed by Me.sup.++ ions in LiF and NaF crystals; F.sub.2.sup.+ ** -F.sub.2.sup.+ centers, perturbed by O-ions in LiF, NaF, NaCl, KCl and KBr crystals; (F.sub.2.sup.+).sub.A -F.sub.2.sup.+ centers, perturbed by Li in NaF and KCl crystals; and (F.sub.2).sub.A -F.sub.2 centers, perturbed by Na ions in CaF.sub.2 and SrF.sub.2 crystals. The second subgroup includes crystals having optical centers, which provide laser properties, represented by impurity ions, perturbed by a neighboring color centers. For example, Cr.sup.4+ doped YAG crystals (i.e., YAG:Cr.sup.4+) likely fall within this subgroup. Other crystal that fall into this group include RbMgF.sub.3 and KMgF.sub.3 crystals that are doped with Mn.sup.2+. These crystals show enormous, but temporary, oscillator strength, which increases after ionizing irradiation.
The fourth, and last, group of active crystalline luminescent media includes impurity doped crystals that have no laser properties in their primal state after crystal growth and that acquire laser properties only after subsequent processing. In these crystals, ionizing irradiation causes metal ion impurities to permanently change their electronic state and valence structure to provide laser properties. Active crystalline media that fall within this category include some of the first laser crystals such as CaF.sub.2 :Sm.sup.2+, CaF.sub.2 :Dy.sup.2+ and CaF.sub.2 :Tm.sup.2+. These crystals may be produced utilizing a procedure of additive coloration or ionizing radiation to transform the rare-earth impurity (Re) from a Re.sup.3+ ion impurity to a Re.sup.2+ ion impurity. Other crystals that fall within this category are SrF.sub.2 crystals doped with neodymium ions that have been treated with ionizing irradiation. The ionizing radiation transforms a portion of the Nd.sup.3+ ion impurities in the SrF.sub.2 crystals into Nd.sup.2+ ion impurities. SrF.sub.2 :Nd.sup.2+ active crystalline media have adequate nonlinear saturation properties for laser system purposes in the infrared range of the optical spectrum from 1.1 .mu.m to 1.6 .mu.m wavelengths. SrF.sub.2 :Nd.sup.2+ crystals were the first solid state passive Q-Switches and mode lockers for resonators of solid state lasers in the above mentioned spectral range.
Prior active crystalline luminescent media for tunable laser systems have numerous disadvantages. For example, laser oscillation for many of these crystals may be suppressed by competing processes, such as: (1) optical bleaching and optical center degradation due to irradiation from the laser pumping system; (2) spontaneous thermal or thermo-optical degradation of the optical center at room temperature; (3) the presence of parasitic absorption in the emission region; (4) absorption in the emission region induced by pump radiation; (5) nonlinear losses due to the excited state absorption; (6) losses from triplet state formation; and (7) overlapping of absorption bands from different optical centers.
Among prior active crystalline luminescent media for tunable laser systems, Ti.sup.3+ doped sapphire crystals (i.e., Ti:Al.sub.2 O.sub.3 crystals) are practically free from the above-mentioned shortcomings and feature advantageous spectroscopic characteristics. In particular, because of the absence of excited state absorption, which is due to d.sup.1 electron configuration of Ti.sup.3+ ion impurity, a Ti:Al.sub.2 O.sub.3 laser crystal medium can lase over the entire fluorescence region and feature a much higher gain cross section than other transition-metal crystalline media for tunable laser systems. Laser systems utilizing Ti-sapphire active crystalline media have a low sensitivity to the quality of the optical elements within the laser cavity and to the spatial, angular and spectral characteristics of the pump radiation. The spectroscopic and laser characteristics of Ti:Al.sub.2 O.sub.3 crystals is described in P. E. Moulton, "Spectroscopic and Laser Characteristics of Ti:Al.sub.2 O.sub.3 ", J. Opt. Soc. Am. B., vol. 3, no. 1, at 125-33 (January 1986).
In an electron paramagnetic-resonance ("EPR") study of the dynamic Jahn-Teller effect, the electron configurations of CaF.sub.2 and SrF.sub.2 crystals having Sc ion impurities were analyzed. In these studies, a CaF.sub.2 crystal and a SrF.sub.2 crystal were plated with Sc metal by condensation from Sc vapor. After processing to allow for diffusion of Sc ions into the crystals, Sc.sup.3+ ion impurity concentrations of about 120 parts per million for CaF.sub.2 and 30 parts per million for SrF.sub.2 were apparently obtained at some point in the crystals. The crystals were X-irradiated at room temperature. EPR spectrum analysis was used to prove the existence of Sc.sup.2+ ion impurities at cubic sites of the crystal lattice for these crystals. No optical or luminescent data could be obtained. This processing method of diffusing Sc ions into the crystal is not usable for preparing laser active elements. This processing yields an extremely low and nonhomogeneous distribution of Sc ions and, therefore, these crystals are not laser active. A discussion of this study is described in U. T. Hochli, "Jahn-Teller Effect of a d.sup.1 Ion in Eightfold Cubic Coordination," Phys. Rev., vol. 162, no. 2, at 262-73 (October 1967); U. T. Hochli, "Paramagnetic-Resonance Study of the Dynamic Jahn-Teller Effect In CaF.sub.2 :Sc.sup.2+ and SrF.sub.2 :Sc.sup.2+," Phys. Rev. Lett., vol. 18, no. 4, at 128-30 (January 1967).
In summary, there are a great variety of crystalline hosts having dopant ions and/or intrinsic color centers that may yield active crystalline luminescent media having adequate properties for use in tunable laser systems. A practical active crystalline luminescent media would be based upon crystals that: (1) have a high concentration of the active optical centers that are both photo-stable and thermo-stable; (2) can be grown to a size and optical quality sufficient to produce energies and beam qualities needed for most applications; (3) feature laser operation at or above room temperatures; and (4) feature a sufficiently large stimulated emission cross section that is much larger than possible parasitic losses and excited state absorption.
The goal of the present invention is to combine the advantages of the technologically well developed and relatively inexpensive fluoride and chloride doped crystals and subsequent ionizing treatment of the crystals to develop practical active crystalline luminescent media, particularly for use in lasers tunable in the UV-visible range of the optical spectrum.